1. Field of the Invention
This invention relates to the intrusion alarm art, and more particularly to a security screen arrangement useful in intrusion alarm systems to secure openings in a building from unauthorized entry.
2. Description of the Prior Art
There have heretofore been proposed security bug screen arrangements having conductive portions connectible in series with a sensing circuit of an intrusion alarm system such that the alarm system will produce a warning signal when the continuity of the conductive portion is broken.
One such prior art security screen arrangement is that disclosed in U.S. Pat. No. 3,051,935 issued to Willson. The protective screen disclosed therein incorporates a series of electrically conductive parallel wire strands interwoven into the screen material itself. The wire strands have substantially the same external diameter and general appearance as the individual strands making up the screening material. The various wire strands are then connected to each other at their respective ends to form at least one continuous conductive circuit connectible in a series relationship with a sensing circuit of an intrusion alarm system to cause the alarm system to produce a warning signal when the conductive loop is broken. The Willson screen is disadvantageous primarily because the conductive strands are interwoven with the screening material. That feature precludes retrofitting existing screens with a conductive loop without replacing the screening material. Further, the ends of the parallel wire strands must be interconnected by hand with relatively short lengths of wire to form the desired continuous conductive loop. The process of manufacture of the Willson device therefore involves relatively high labor costs, precludes retrofit of existing screens and results in a configuration having a large number of splices susceptible to corrosion and malfunction.
Stitching a continuous loop of conductive wire onto the screening material has also been proposed as a method of manufacturing security screens to serve the instant purpose. While a stitching process can be used in a retrofit program, it has been found to cause puckering of the mesh screening material and sometimes damage thereof. A stitching process must also be performed on a sewing machine with the screening material off the screen frame, and can be very time consuming.
An embellishment which has been incorporated in some security screen arrangements to guard against removal of the entire screen from the opening of a building is an electrical switch which is placed in series with both the conductive wire loop and the sensing circuit of the intrusion alarm to open that circuit when the screen is removed. A magnetic reed switch, for example, has been placed in the splining used to retain the screening material within its frame such that the switch will be located adjacent a permanent magnet in a relation holding the switch closed for the screen in position over the opening. Removal of the entire screen from the opening therefore allows the switch to open, actuating the alarm. However, this configuration results in a variety of problems. One major problem is the susceptibility of the circuit to damage from corrosion, particularly at the points of electrical connection between the wires in the screen and the leads to the switch. The wiring for the switch is located in or adjacent to the exposed splining and is not effectively sealed from the elements. Moisture from the weather or from cleaning the screen is thus able to reach the points of electrical connection to corrode or short them out. Location of the glass encapsulated switch within the spline also causes it to be susceptible to damage from bending or impact of the screen frame. Perhaps the most serious drawback of this configuration is the ease with which the alarm can be circumvented. The splining, switch, screening material and all of the wiring can simply be removed together from the screen frame with the electrical circuit intact, allowing an intruder to pass through the empty frame undetected.
Another switch proposed for this purpose comprises a pair of contacts on the screen frame which are designed to be shorted by a conductive bar when the screen is in position over the opening. This switch is disclosed in the Willson patent discussed above. Such contacts, however, are exposed and therefore subject to corrosion which may interfere with the conduction of electricity thereacross. Exposed switches of any sort should also be avoided in intrusion alarms generally to reduce the possibility of circumvention by an intruder.
Therefore, in many applications it is desirable to provide a security screen arrangement which may be fabricated easily and economically either in the initial manufacture or retrofit of mesh bug screens, and in which the circuitry is as tamper-proof and corrosion resistant as possible.